Many ink jet printers use DC servo motors to drive the movable carriage that contains the ink jet printhead. DC servo motors have been used for many years for driving variable-speed devices, or for driving low power constant-speed devices that run from direct current, rather than from alternating current. However, stepper motors have become much more prevalent in many low-power applications for not only variable positioning devices, but also for variable-speed or constant-speed devices. Consequently, the integrated circuits that control stepper motors have become much less expensive, due to their much greater quantity of production, than integrated circuits that control DC servo motors.
While stepper motor driver circuits are less expensive than DC servo motor driver circuits, these devices are certainly not interchangeable with one another, and also their actual motor constructions are not interchangeable with one another. Stepper motors require control of two separate windings, and the current through each of these windings must be controllable in both directions by the driver circuitry. Moreover, stepper motors have been very useful for exact positioning applications, and have not completely replaced DC servo motors for certain applications, such as constant-speed drives using a low power DC power supply.
DC servo motors are typically driven with an electronic circuit that includes an "H-bridge" output driver stage, which is illustrated in FIG. 1 in some detail, and will be described in greater detail hereinbelow in the "Detailed Description." The branches of the circuit that make up the "H-bridge" comprise the four output transistors and the DC servo motor, which comprises the cross-bar of the "H." The method in which the four transistors are controlled determines whether or not the DC servo motor turns in its forward direction or in its reverse direction.
It would be commercially advantageous to be able to drive a DC servo motor while using a lower cost integrated circuit than the commonly-used L6202 integrated circuit, which is manufactured by SGS-Thomson Microelectronics.